Problem: Solve the equation. $ 12 = n - 9$ $n=$
Solution: Add $9$ to both sides: 12 + 9 12 + 9 = = n n − 9 + 9 \begin{eqnarray} 12 &=& n &- 9 \\ \\ {+9} &&& {+9} \\\\ 12 {+ 9} &=& n \end{eqnarray} Simplifying, we get: $ n = 21$